JP2023061620A - Martensitic stainless steel sheet - Google Patents

Abstract

To provide a martensitic stainless steel sheet that has excellent rusting resistance.SOLUTION: Adopted is a martensitic stainless steel sheet that contains C, Si, Mn, P, S, Cr, Ni, Cu, N, Mo, V, O, Al, Mg and Ca, and the balance being Fe and impurities, and in which, on the surface of the steel sheet, among the oxysulfide having a circle equivalent diameter of 1 μm or more and containing S by 5% or more, the number ratio of (Mn,Cr)S-based oxysulfide is 70% or more and the number ratio of CaS-based oxysulfide is less than 30%, and, on the surface of the steel sheet, the number density of oxysulfide having a circle equivalent diameter of 5 μm or more and containing S by 5% or more is 0.50/mm2 or less.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a martensitic stainless steel sheet having excellent rust resistance.

Stainless steel is generally put to practical use without being coated or the like. Martensitic stainless steel is a type of steel with a high carbon concentration to improve quenching hardness. Applications include cutlery products such as Western table knives (table knives) and scissors, tools such as loom parts and vernier calipers, and two-wheel disc brakes. and structural members such as reinforcing bars. In such applications, it is difficult to form a plating layer for rust prevention, apply a coating, or apply an antirust oil.

In general, the corrosion resistance of a stainless steel base material is greatly affected by its components, and is organized by a pitting resistance equivalent (PRE) or the like. The higher the numerical value, the higher the corrosion resistance. Corrosion resistance in this case refers to a neutral chloride aqueous solution environment, and evaluation methods include, for example, the pitting corrosion potential measurement method for stainless steel specified in JIS G0577 and the salt spray test method specified in JIS Z2371. etc. are used. However, there are cases where the corrosion resistance assumed from the composition of the base material does not correspond to the actual corrosion resistance of the steel. Typical causes of corrosion resistance deterioration in this case include the presence of CaS, which is a sulfide exposed on the steel material surface, and the presence of a Cr-depleted layer (sensitization) formed around carbides during quenching.

Patent Document 1 describes a Ca-containing steel that suppresses CaS generated when the temperature of molten steel decreases or during solidification by controlling the equilibrium S concentration of inclusions in molten steel to a low level.

In Patent Document 2, by controlling the CaO concentration in the slag at the end of refining to 35% or less, the accumulation of S in CaO is suppressed, and the MgO concentration in the slag is controlled to 30% or less. , describes a method for producing high-Al stainless steel with excellent rust resistance in which solid-phase MgO having good lattice matching with CaS is generated to prevent CaS from easily precipitating.

In Patent Document 3, the value of the formula regarding the composition of inclusions represented by the X value is set to a certain value or less, and [Ca], [S], [Al], T.I. It describes a ferritic stainless steel that suppresses the generation of CaS by refining so as to satisfy the formula of [O] and generates little rust.

Patent Literature 4 describes a martensitic stainless steel sheet that avoids sensitization during quenching and has excellent manufacturability and corrosion resistance in which oxides with a size of 10 μm or more are reduced to 0.2 pieces/cm ² or less. In Patent Document 4, Al is reduced to 0.02% or less and O is reduced to 0.001 to 0.01% in order to reduce the amount of inclusions generated.

Japanese Unexamined Patent Application Publication No. 2001-107178 JP-A-5-339620 JP 2014-162948 A JP 2018-9231 A

In Patent Literature 1, since CaS generated during heating of the cast slab before hot rolling is not taken into consideration, corrosion resistance may deteriorate.

The techniques of Patent Documents 2 and 3 have many restrictions on the slag composition and the concentration of Ca and S in molten steel. In addition, in martensitic stainless steel, solid-solution strengthening impairs toughness, so it is necessary to reduce the Al concentration as much as possible. Therefore, there is room for further improvement.

Furthermore, in the technique described in Patent Document 4, rust generation itself becomes less conspicuous due to the reduction of coarse inclusions. However, the composition of sulfides generated after the refining process cannot be controlled, and there is room for further improvement in corrosion resistance.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a martensitic stainless steel sheet having excellent rust resistance.

The present inventors conducted a detailed investigation of the factors that affect the rust resistance of high C content martensitic stainless steel sheets produced by various methods. As a result, it was found that the composition and size of sulfides affected the rust generation in steel sheets subjected to normal quenching treatment. In this specification, sulfides are often formed around or inside oxides and are difficult to distinguish, so inclusions containing 5% or more of S are referred to as oxysulfides.

That is, on the surface of the steel sheet, the number of oxysulfides with an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50/mm ² or less, and the number of oxysulfides with an equivalent circle diameter of 1 μm or more ( It has been found that when the number of Mn, Cr)S-based inclusions is 70% or more with respect to the CaS-based inclusions, the rust resistance is improved.

The present invention has been made based on the above findings in order to solve the above problems, and the gist thereof is as follows.

[1] in % by mass,
C: 0.10 to 0.60%,
Si: 0.05 to 1.0%,
Mn: [S] × 100% to 1.0% (where [S] is the S content (%)),
P: 0.04% or less,
S: 0.008% or less,
Cr: 11-16%,
Ni: 0.01 to 0.50%,
Cu: 0.01-0.50%,
N: 0.01 to 0.10%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.5%,
O: 0.0010 to 0.0080%,
Al: 0.001% to 0.025%,
Mg: 0.0020% or less,
Ca: containing 0.0020% or less,
The balance consists of Fe and impurities,
On the surface of the steel sheet, among the oxysulfides having an equivalent circle diameter of 1 μm or more and containing 5% or more of S, the number ratio of (Mn, Cr) S-based oxysulfides is 70% or more, and the number ratio of CaS-based oxysulfides is 70% or more. The number ratio is less than 30%,
A martensitic stainless steel sheet, wherein the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm ² or less on the surface of the steel sheet.
[2] in % by mass,
C: 0.10 to 0.60%,
Si: 0.05 to 1.0%,
Mn: [S] × 100% to 1.0% (where [S] is the S content (%)),
P: 0.04% or less,
S: 0.008% or less,
Cr: 11-16%,
Ni: 0.01 to 0.50%,
Cu: 0.01-0.50%,
N: 0.01 to 0.10%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.5%,
O: 0.0010 to 0.0080%,
Al: 0.001% to 0.025%,
Mg: 0.0020% or less,
Ca: 0.0020% or less,
Containing Ta: 0.0005 to 0.01%,
The balance consists of Fe and impurities,
(Mn, Cr) S-based oxysulfides and (Mn, Cr, Ta) S-based oxysulfides among the oxysulfides containing 5% or more of S and having an equivalent circle diameter of 1 μm or more on the surface of the steel sheet The total number ratio is 70% or more, and the CaS-based oxysulfide number ratio is less than 30%,
A martensitic stainless steel sheet, wherein the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm ² or less on the surface of the steel sheet.
[3] In place of part of Fe, Co: 0.05 to 1.00%, Ti: 0.05% or less, Nb: 0.05% or less, B: 0.005% Hereinafter, Sn: 0.005 to 0.20% or less, REM: 0.002% or less, characterized by containing one or more selected from [1] or [2] Martensitic stainless steel sheet.

According to the present invention, a martensitic stainless steel sheet having excellent rust resistance can be provided.

FIG. 3 is a diagram of the ratio of the rusted area to the ratio of the number of (Mn, Cr)S-based oxysulfides.

The contents of the present invention will be described in detail below.
As described above, the typical causes of deterioration in corrosion resistance of martensitic stainless steel with a high C content are the presence of a Cr-deficient layer (sensitization) formed around carbides during quenching, and the presence of CaS, which is a sulfide exposed on the surface of the steel material. and the existence of

Since sensitization in stainless steel is accelerated as the C content increases, it has been thought that martensitic stainless steel with a high C content is likely to be sensitized. In general, during quenching and heating, in order to prevent the austenite grain size from becoming coarse, it is heated at a temperature at which some chromium carbides do not form a solid solution, leaving undissolved chromium carbides, thereby inhibiting the growth of austenite grains. Let Therefore, undissolved carbides are finely dispersed in the steel after quenching. However, the present inventors have confirmed that these carbides are relatively fine, and do not form a chromium-deficient layer around them during quenching and cooling, and do not impair corrosion resistance.

CaS is an example of a non-metallic inclusion in steel that is a starting point for rust generation. Even though CaS is not present at the slab stage, that is, at the steelmaking stage, CaS may be produced during slab heating prior to hot rolling as described above. The mechanism is that Ca separates from CaO in the inclusions and S in the inclusions also separates, and these react to form CaS, or S in the base metal reacts with CaO in the inclusions. It is thought that CaS is generated by doing so. Therefore, if both CaO in the inclusions and S in the base metal are reduced before heating the slab, CaS will be suppressed.

Here, in martensitic stainless steel with a high C content, ferrite is formed as primary crystals when molten steel is solidified, so C concentrates in the liquid phase and the solidus temperature drops. Therefore, the solidification time is longer than other stainless steels, and the growth of inclusions during solidification is facilitated.

Therefore, in a high-C martensitic stainless steel, when the Ca, Al, and Mg contents are high, CaO—Al ₂ O ₃ —MgO inclusions aggregate and coarsen, which tends to be disadvantageous for CaS suppression. It has been found. On the other hand, the advantage that S in the matrix can be fixed to (Mn,Cr)S during solidification was also revealed. Therefore, many studies were conducted to control the composition of sulfides generated during solidification.

As a result, by optimizing the amount of Mn and S in the steel and controlling various deoxidizing elements and O concentration, CaS is reformed to (Mn, Cr) S, and (Mn, Cr) S It has been found that a steel sheet having excellent rust resistance which is controlled in a small amount can be obtained. When the Mn concentration and the oxygen concentration are high, MnO.Cr ₂ O ₃ is generated first, and S in the base material cannot be fixed.

<About steel composition>
As described above, the present invention relates to inclusion composition control and composition control of martensitic stainless steel, and is applicable to martensitic stainless steel that is generally produced. Although the range of components that can be suitably used is shown below, it is not limited to this.

C: 0.10-0.60%
C is necessary for lowering the solidus temperature to promote the growth of inclusions and for obtaining a martensite structure after quenching treatment to obtain high strength. Therefore, the C content should be 0.10% or more. Preferably, it is 0.20% or more. On the other hand, if an excessive amount of C is contained, coarse carbides are formed, which act as rust initiation points, so the C content is made 0.60% or less. Preferably, it is 0.55% or less.

Si: 0.05-1.0%
Si is contained in order to reduce and recover Cr oxides produced during decarburization of stainless steel. Therefore, the Si content should be 0.05% or more. For deoxidation, the content should preferably be 0.10% or more. On the other hand, when Si exceeds 1.0%, impurities in the Si raw material and slag are reduced during refining to increase Ca in the molten steel, so the Si content is made 1.0% or less. In order to narrow the austenite single-phase temperature range and impair the quenching stability, the content is preferably 0.80% or less.

Mn: [S] × 100% to 1.0% (where [S] is the S content (mass%) in the steel)
Mn is an element that fixes S together with Cr and suppresses CaS, and also contributes to the improvement of hardenability by expanding the austenite single-phase region. Therefore, the Mn content is set to [S]×100% or more. It is preferably [S]×120% or more. On the other hand, when the Mn content exceeds 1.0%, the generation of MnO.Cr ₂ O ₃ inhibits the generation of (Mn, Cr) S and promotes the generation of oxide scale during heating for quenching. The upper limit is 1.0% or less. Considering the coarsening of (Mn, Cr) S, it is desirable to make it 0.8% or less.

P: 0.04% or less P is an element contained as an impurity in the main raw material such as hot metal and ferrochromium.
Since it is an element harmful to the toughness and corrosion resistance of a hot-rolled and annealed sheet and quenched steel, its content is made 0.04% or less. In addition, it is preferably 0.030% or less.

S: 0.008% or less S forms sulfide-based inclusions, deteriorating ^the corrosion resistance of the steel material. The upper limit of the content is preferably as small as possible, and the upper limit is made 0.008%. Preferably, it is 0.004% or less.

Cr: 11-16%
Cr is an important element that provides corrosion resistance to stainless steel, and together with Mn, it fixes S and suppresses CaS. Therefore, at least 11% or more is required. Preferably it is 12% or more. On the other hand, in order to prevent the formation of residual ferrite after quenching, the upper limit is set at 16%. Preferably, it is 15% or less.

Ni: 0.01-0.50%
Ni stabilizes austenite without causing solidification segregation. In addition, other austenite stabilizing elements such as C, N, and Mn may decrease from the surface layer due to decarburization, denitrification, and oxidation during quenching and heating, and may form ferrite in the surface layer. It is an important element that does not decrease. Ni is an element that is also effective in suppressing the progress of pitting corrosion, and the effect is exhibited from 0.01%, so the content is made 0.01% or more. On the other hand, in order to raise the solidus temperature and suppress the growth of inclusions, the upper limit is made 0.50% or less. A large amount of Ni is preferably 0.30% or less because there is a possibility that the press formability of the hot-rolled and annealed sheet will be deteriorated due to solid-solution strengthening. Also, considering the effect of uniforming scale formation during quenching, the lower limit is preferably 0.05% or more.

Cu: 0.01-0.50%
Cu can suppress active dissolution and ensure corrosion resistance, so 0.01% or more is necessary. For steels with a low pitting corrosion resistance index PRE, the content is preferably 0.02% or more as a countermeasure. On the other hand, an excessive content lowers the corrosion resistance and also deteriorates manufacturability such as slab cracking, so the content is made 0.50% or less. Cu is precipitated during quenching and tempering and may impair the soundness of the passive film, thereby impairing the corrosion resistance.

N: 0.01-0.10%
N, like C, is necessary in order to lower the solidus temperature, promote the growth of inclusions, and raise the quenching hardness. It also has the effect of strengthening the passive film and suppressing the precipitation of Cr carbide (suppressing the Cr-deficient layer). In order to obtain these effects, N should be 0.01% or more. However, an excessive content is 0.10% or less because it causes blowholes. In order to suppress sensitization, N is preferably 0.02% or more. Moreover, since N increases the hardness of the hot-rolled annealed sheet and lowers the workability, it is preferably 0.05% or less.

Mo: 0.01-1.0%
Mo is an element necessary for improving the corrosion resistance of the sensitized portion, so its content is made 0.01% or more. 0.02% or more is preferable for suppressing the progress of pitting corrosion. On the other hand, it is an element that stabilizes the ferrite phase, and is made 1.0% or less in order to avoid acceleration of sensitization accompanying formation of residual ferrite. The content is preferably 0.8% or less in order to increase the resistance to temper softening and increase the annealing time of the hot-rolled sheet, thereby degrading the manufacturability.

V: 0.01-0.5%
V not only has the effect of improving corrosion resistance, but also has the effect of finely dispersing the precipitation of V when it dissolves in carbonitrides, so the lower limit is made 0.01% or more. It is preferably 0.02% or more. Since it has a strong effect of narrowing the austenite single-phase temperature range, it is made 0.5% or less. In order to avoid deterioration in toughness due to coarsening of precipitates, the content is preferably 0.20% or less.

O: 0.0010 to 0.0080%
O is made 0.0080% or less in order to suppress the generation of coarse inclusions including CaO-based inclusions and MnO.Cr ₂ O ₃ . It is preferably 0.0050% or less. However, due to excessive deoxidation, Ca, Al, and Mg are likely to be mixed into the molten steel from the slag during refining, so the content is made 0.0010% or more. It is preferably 0.0020% or more.

Al: 0.001% to 0.025%
Al is included for deoxidation and desulfurization. In addition, CaO-based inclusions contain Al ₂ O ₃ to reduce the solid-solution S concentration, thereby making it difficult to precipitate CaS. Therefore, the lower limit is made 0.001% or more. 0.002% or more is preferable in order to stabilize deoxidation and desulfurization. However, an excessive content impairs toughness due to solid solution strengthening, so the upper limit is made 0.025% or less. In order to avoid reducing Ca and Mg from slag to molten steel during refining, the content is preferably 0.020% or less. More preferably, it is 0.015% or less.

Mg: 0.0020% or less Mg is an element effective for deoxidizing and desulfurizing, but excessive inclusion forms coarse inclusions including CaO inclusions. Also, it combines with Al ₂ O ₃ in CaO-based inclusions to form MgO.Al ₂ O ₃ . As a result, the ratio of CaO inclusions relatively increases, increasing the upper limit of the solid solution S concentration, thereby facilitating precipitation of CaS. Therefore, the upper limit is made 0.0020% or less. It is preferably 0.0010% or less. The lower limit of Mg may be 0.00005% or more, 0.0001% or more, or 0.0002% or more.

Ca: 0.0020% or less Ca is an element that is effective for deoxidizing and desulfurizing, but when it is excessively contained, it causes the formation of coarse inclusions including CaO-based inclusions and facilitates the precipitation of CaS. Therefore, the upper limit is made 0.0020% or less. It is preferably 0.0010% or less. The lower limit of Ca may be 0.00005% or more, 0.0001% or more, or 0.0002% or more.

The rest of the above steel components are Fe and impurities. The term "impurities" as used herein refers to components that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when steel is manufactured industrially, and are within a range that does not adversely affect the present invention. permissible in

In the present invention, Ta can be contained in addition to the essential elements explained above.

Ta: 0.0005-0.01%
Ta is an element effective for deoxidation and desulfurization, and improves corrosion resistance by dissolving in (Mn, Cr)S to form (Mn, Cr, Ta) S. In order to obtain this effect, the content of 0.0005% or more is required. It is preferably 0.0010% or more. However, an excessive content causes a decrease in room temperature ductility and a decrease in toughness, so the upper limit is made 0.01% or less. It is preferably 0.005% or less.

Furthermore, in the present invention, in addition to the elements explained above, one or more of Co, Ti, Nb, B, Sn and REM may be included.

Co: 0.05-1.00%
Co has the effect of increasing the strength of the steel material, so it can be contained as necessary. In order to obtain this effect, the content must be 0.05% or more. However, since excessive content causes a decrease in toughness, the upper limit is made 1.00% or less.

Ti: 0.05% or less Ti is an element that suppresses sensitization and deterioration of corrosion resistance due to precipitation of chromium mononitride by forming carbonitrides. In order to obtain this effect, the content is preferably 0.001% or more.
However, if it is excessively contained, coarse oxides are formed and the toughness is lowered, so the upper limit is made 0.05% or less.

Nb: 0.05% or less Nb is an element that suppresses sensitization and deterioration of corrosion resistance due to precipitation of chromium mononitride by forming carbonitrides. In order to obtain this effect, the content is preferably 0.001% or more.
However, an excessive content destabilizes the martensite phase and lowers the hardness, so the upper limit is made 0.05% or less.

B: 0.005% or less B is an element effective in improving hot workability. In order to obtain this effect, the content is preferably 0.0002% or more. However, if it is excessively contained, it precipitates together with carbides and deteriorates the hardenability, so the upper limit is set to 0.005% or less.

Sn: 0.005% to 0.20% or less Sn is a segregating element, and is concentrated at the interface between the matrix and the precipitates to suppress the growth and coarsening of the precipitates. Therefore, since the effect of suppressing sensitization and improving corrosion resistance can be obtained, the content of 0.005% or more is preferable. However, Sn has a low solid solubility limit in the austenite phase and is known to cause hot rolling cracks and flaws in ordinary steel. is preferred.

REM: 0.002% or less REM (rare-earth metal) has a high affinity with O and S, so it is an effective element for deoxidizing and desulfurizing, and may be contained as necessary. However, if added excessively, a large amount of oxides will form before or during casting, causing casting troubles such as nozzle clogging and the formation of coarse inclusions, so the upper limit is made 0.002% or less. REM indicates a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of REM means the total content of these 17 elements.

<Method for measuring oxysulfide>
A method for measuring inclusions in a steel sheet will be described below. A mirror finish is applied to observe the surface of the steel plate.

(Number density of oxysulfides)
On the observed surface, inclusions having an equivalent circle diameter of 5 μm or more are randomly analyzed by SEM-EDS to identify the composition of the inclusions. An inclusion containing 5% or more of S is defined as an oxysulfide. At least 10 oxysulfides are measured in an observation area of 100 mm ² or more. If the number of oxysulfides having an equivalent circle diameter of 5 μm or more is less than 10 in the set observation area, the observation area is enlarged. In this way, the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing S of 5% or more is measured.

(Number ratio of oxysulfides)
Next, 100 or more oxysulfides having an equivalent circle diameter of 1 μm or more and containing 5% or more of S are arbitrarily selected, and the composition of inclusions is measured by an EDS (energy dispersive elemental analyzer). Then, (Mn, Cr) S-based oxysulfides, (Mn, Cr, Ta) S-based oxysulfides, and CaS-based oxysulfides are classified based on the criteria described later, and the number of each is measured. Ask for a percentage.

In addition, according to JIS G0555, which is generally used as an evaluation method for inclusions, even when two or more inclusions exist apart from each other, they may be regarded as one inclusion depending on the type and distance. are regarded as individual inclusions.

Next, the reasons for limiting the number of oxysulfides and the relationship between the composition of the sulfide portion and the corrosion resistance will be explained.

<The number of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50/mm ² or less>
The more sulfides formed around and inside the coarse oxides, the more likely they are to be observed as rust. Investigation of the relationship between inclusions before the corrosion resistance test and the rusted part after the test shows that there is a high probability of rust originating from oxysulfides with an equivalent circle diameter of 5 μm or more. Targets sulfides. If a large number of oxysulfides having an equivalent circle diameter of 5 μm or more ^are present, the number of rust initiation points increases and the rust resistance deteriorates. Preferably, it is 0.40/mm ² or less.

<The number ratio of (Mn, Cr) S-based oxysulfides having an equivalent circle diameter of 1 μm or more is 70% or more in the oxysulfides>
It is difficult to determine the composition of sulfides because they are formed around or inside oxides. The concentrations (% by mass) of MnS, CrS, and CaS are calculated from the concentrations (% by mass) of S, Mn, Cr, and Ca in each oxysulfide. Inclusions with a (Mn, Cr) S/CaS ratio exceeding 1 and inclusions with a CaS content of 0 are defined as (Mn, Cr) S-based oxysulfides. Other oxysulfides are referred to as CaS-based oxysulfides. Then, each number is measured. If 70% or more of the oxysulfide is (Mn, Cr) S-based oxysulfide, and CaS-based oxysulfide is less than 30%, S in the base material is fixed and CaS-based oxysulfide is suppressed. judged to have been As shown in FIG. 1, by setting the number ratio of (Mn, Cr) S-based oxysulfides to 70% or more in the oxysulfides, the rust area ratio can be reduced to 10% or less, and a steel sheet having excellent rust resistance can be obtained. Obtainable. The (Mn,Cr)S-based oxysulfide in the oxysulfide is preferably 80% or more. In FIG. 1, the rust area ratio is 10% or more (X), and the rust area ratio of less than 10% is (O).

<The total number ratio of (Mn, Cr) S-based oxysulfides and (Mn, Cr, Ta) S-based oxysulfides having an equivalent circle diameter of 1 μm or more is 70% or more in the oxysulfides>
When Ta is contained in steel, (Mn, Cr, Ta) S-based oxysulfides are produced in addition to (Mn, Cr) S-based oxysulfides. Ta is a highly reducing element, and when Ta is contained in the (Mn, Cr)S-based oxysulfide, the corrosion resistance can be further improved. The concentrations (% by mass) of MnS, CrS, TaS and CaS are calculated from the concentrations (% by mass) of S, Mn, Cr, Ta and Ca in each oxysulfide. Inclusions with a (Mn, Cr, Ta) S/CaS ratio exceeding 1 and inclusions with a CaS content of 0 are defined as (Mn, Cr, Ta) S-based oxysulfides. Inclusions not containing Ta are assumed to be (Mn, Cr) S-based oxysulfides. Others are CaS-based oxysulfides. Then, each number is measured. If 70% or more of the oxysulfide is (Mn, Cr) S-based oxysulfide and (Mn, Cr, Ta) S-based oxysulfide, and the CaS-based oxysulfide is less than 30%, the base material It is determined that the S of is fixed and the CaS-based oxysulfide is suppressed. The content of (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides in the oxysulfides is preferably 80% or more.

<CaS-based oxysulfides with an equivalent circle diameter of 1 μm or more are less than 30% in the oxysulfides>
Even if no (Mn,Cr)S or (Mn,Cr,Ta)S are formed in the steel at the slab stage, CaS can be formed during slab heating before hot rolling . Therefore, (Mn, Cr) S-based oxysulfides and (Mn, Cr, Ta) S in which the ratio of (Mn, Cr) S / CaS or the ratio of (Mn, Cr, Ta) S / CaS is 1 or less CaS-based oxysulfides are used as oxysulfides other than the CaS-based oxysulfides, and the number ratio is limited. If the CaS-based oxysulfide in the oxysulfide is 30% or more, rust cannot be suppressed and the rust area ratio increases, which is undesirable. The CaS-based oxysulfide in the oxysulfide is preferably 20% or less.

<Manufacturing method>
The martensitic stainless steel sheet of the present embodiment is a hot-rolled steel sheet or a cold-rolled steel sheet usually produced using a method for producing a martensitic stainless steel sheet. Hot-rolled steel sheets are manufactured by melting/casting-hot-rolling-hot-rolled sheet annealing/pickling, and cold-rolled steel sheets are subsequently manufactured by cold-rolling-cold-rolled sheet annealing/pickling.

First, steel adjusted to have the above-described predetermined components is melted. In addition to reducing the S concentration, by controlling various deoxidizing elements such as Al, Mg, and Si, oxygen concentration, and elements that form oxysulfides such as Ca, Mn, and Cr, oxysulfide with an equivalent circle diameter of 5 μm or more The number density of objects can be reduced to 0.50/mm ² or less.

In addition, the temperature difference ΔT0 (solidification temperature range) between the liquidus temperature (T _LL ) and the solidus temperature (T _SL ) is 70° C. or more, and the (Mn, Cr) S-based oxysulfide or (Mn, Cr) , Ta) can generate an S-based oxysulfide. The solidification temperature range ΔT0 can be easily calculated from the steel composition using a thermodynamic data collection or commercial thermodynamic calculation software.

In addition to the above, casting is performed by controlling the cooling rate. By controlling the average cooling rate between 1400 to 700 ° C. in the vicinity of the slab surface to 50 ° C./min or less, (Mn, Cr) S-based oxysulfide or (Mn, Cr, Ta ) The ratio of S-based oxysulfide can be 70% or more. A preferable range of the average cooling rate is 30° C./min or less, and a more preferable range is 15° C./min or less.

After casting, a steel plate may be manufactured under known manufacturing conditions. For example, after heating the slab at 1100 to 1300° C., it is finished to a plate thickness of 2 to 8 m by rough rolling and finish rolling. In the hot-rolled sheet annealing step, annealing is performed at 750 to 900° C. using ordinary box annealing. Pickling is performed to remove scales on the surface to obtain a hot-rolled steel sheet. In the case of cold-rolled steel sheets, cold-rolling and final annealing are subsequently performed to produce products.

EXAMPLES The effects of the present invention will be described below with reference to examples, but the present invention is not limited to the conditions used in the examples below.

In this example, steels having chemical compositions shown in Tables 1A to 1D were melted and continuously cast into slabs having a thickness of 200 mm. At this time, casting was performed while controlling the cooling rate of the slab at various rates. The average cooling rate at 1400 to 700°C in the vicinity of the slab surface was evaluated by numerical calculation using heat transfer analysis, and is shown in Tables 2 and 3. After heating the slab at 1150° C. to 1250° C. for 2 hours, hot rolling was performed to obtain a hot rolled steel sheet having a thickness of 5 mm. The hot-rolled sheet was annealed at 850° C., pickled, and cold-rolled to a sheet thickness of 3.0 mm. This thin sheet coil was finally annealed and pickled to obtain a cold-rolled sheet as a product sheet.

Inclusions were measured as follows. First, the steel plate surface was mirror-finished for observation.

Inclusions having an equivalent circle diameter of 5 μm or more on the mirror-finished observation surface were randomly analyzed by SEM-EDS to identify the composition of the inclusions. An inclusion containing 5% or more of S was defined as an oxysulfide. At least 10 oxysulfides were measured in an observation area of 100 mm ² . When the number of oxysulfides having an equivalent circle diameter of 5 μm or more was less than 10 in the set observation area, the observation area was enlarged. In this way, the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing S of 5% or more was measured.

Next, 100 or more oxysulfides having an equivalent circle diameter of 1 μm or more and containing 5% or more of S were arbitrarily selected, and the composition of inclusions was measured by an EDS (energy dispersive elemental analyzer). Then, (Mn, Cr) S-based oxysulfides, (Mn, Cr, Ta) S-based oxysulfides and CaS-based oxysulfides are classified based on the criteria described later, the number of each is measured, and the number ratio is calculated. asked for In Tables 2 and 3, the column "ratio of (Mn, Cr) S-based oxysulfides" includes (Mn, Cr) S-based oxysulfides and (Mn, Cr, Ta) S-based oxysulfides. The total ratio of

According to JIS G 0555:2015, which is generally used as an evaluation method for inclusions, even when two or more inclusions are separated, they may be regarded as one inclusion depending on the type and distance. They were considered individual inclusions in the examples.

(When it does not contain Ta)
The concentrations (% by mass) of MnS, CrS, and CaS were calculated from the concentrations (% by mass) of S, Mn, Cr, and Ca in each oxysulfide, and the ratio of (Mn, Cr) S/CaS was Inclusions with more than 1 CaS and inclusions with 0 CaS were defined as (Mn, Cr) S-based oxysulfides. Other oxysulfides were used as CaS-based oxysulfides.

(When containing Ta)
The concentrations (% by mass) of MnS, CrS, TaS and CaS were calculated from the concentrations (% by mass) of S, Mn, Cr, Ta and Ca in each oxysulfide. Inclusions with a (Mn, Cr, Ta) S/CaS ratio exceeding 1 and inclusions with a CaS content of 0 were defined as (Mn, Cr, Ta) S-based oxysulfides. CaS-based oxysulfides were used other than (Mn, Cr, Ta) S-based oxysulfides and (Mn, Cr) S-based oxysulfides. Steel no. In A28, Table 2 shows the total number ratio of (Mn,Cr)S-based oxysulfides and (Mn,Cr,Ta)S-based oxysulfides.

Evaluation of rust resistance was performed by a salt spray test based on JIS Z 2371:2015 after polishing with #600. A 24-hour continuous spray test was performed using a neutral salt spray test as the salt solution. A rust area ratio of 10% or more was rejected (x), and a rust area ratio of less than 10% was accepted (o). In addition, those with a rust area ratio of 5% or less were regarded as good materials (⊚).

As shown in Tables 1A, 1B and 2, no. In the present invention examples A1 to A30, the chemical composition of the steel and the number density and number ratio of oxysulfides satisfied the ranges of the present invention, and the rust resistance was excellent.

On the other hand, as shown in Tables 1C, 1D and 3, No. Comparative examples a1 to a27 did not satisfy the range of the present invention in either the chemical composition of the steel, the number density or the number ratio of oxysulfides, and were inferior in rust resistance.

No. In a1, the C content was small, ΔT0 was less than 70°C, the formation of (Mn, Cr)S-based oxysulfide was not promoted, and the rust-generating property was lowered.
No. In a2, the C content was excessive, coarse carbides were formed, and the rust resistance was lowered.

No. In a3, the Si content was small, MnCr oxides were formed, and the formation of (Mn, Cr)S-based oxysulfides was not promoted, resulting in a decrease in rusting properties.
No. In a4, the Si content was excessive, deoxidation was excessive, Ca in the molten steel increased, CaS-based oxysulfides increased, and rusting decreased.

No. In a5, the Mn content was low, and the formation of (Mn, Cr)S-based oxysulfides was not promoted, resulting in reduced rusting.
No. In a6, the Mn content was excessive, MnCr oxides were formed, and the formation of (Mn, Cr)S-based oxysulfides was not promoted, resulting in a decrease in rusting.

No. In a7, the P content was excessive, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.
No. In a8, the S content was excessive, and a large amount of oxysulfide was generated, resulting in an increase in the number density and a decrease in rusting resistance.

No. In a9, the Cr content was low, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.
No. In a10, the Cr content was excessive, MnCr oxides were formed, and the formation of (Mn, Cr)S-based oxysulfides was not promoted, resulting in reduced rusting.

No. In a11, the Ni content was excessive, and the formation of (Mn, Cr)S-based oxysulfide was not promoted, resulting in a decrease in rust-generating properties.
No. In a12, the Ni content was small, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.

No. In a13, the Cu content was small, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.
No. In a14, the Cu content was excessive, the corrosion resistance of the steel was lowered, and the rusting property was lowered.

No. In a15, the N content was small, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.
No. In a16, the N content was excessive, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.

No. In a17, the Mo content was excessive, and the manufacturability was lowered, and the rust-generating property was lowered.
No. In a18, the Mo content was small, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.

No. In a19, the V content was small, the corrosion resistance of the steel was lowered, and the rusting resistance was lowered.
No. In a20, the V content was excessive, and the precipitates were coarsened, the toughness was lowered, and the rusting resistance was lowered.

No. In a21, the O content was small, a large amount of CaS-based oxysulfide was produced, and the rust-generating property was lowered.
No. In a22, the O content was excessive, MnCr oxides were formed, and the formation of (Mn, Cr)S-based oxysulfides was not promoted, resulting in a decrease in rusting properties.

No. In a23, the Al content was excessive, a large amount of CaS-based oxysulfide was generated, and the rust-generating property was lowered.
No. In a24, the Al content was small, and a large amount of (Mn, Cr)S-based oxysulfides were formed and coarsened, resulting in a decrease in the number density and a decrease in rusting properties.

No. The a25 had a low Ca content, produced a large amount of CaS-based oxysulfide, and had a low rusting property.
No. In a26, the Ca content was excessive, a large amount of CaS-based oxysulfide was produced, and the rusting property was lowered.

No. In a27, the chemical composition satisfied the range of the present invention, but since the slab cooling rate was low, a large amount of CaS-based oxysulfide was formed, and the rusting property was lowered.

Claims (3)

in % by mass,
C: 0.10 to 0.60%,
Si: 0.05 to 1.0%,
Mn: [S] × 100% to 1.0% (where [S] is the S content (%)),
P: 0.04% or less,
S: 0.008% or less,
Cr: 11-16%,
Ni: 0.01 to 0.50%,
Cu: 0.01-0.50%,
N: 0.01 to 0.10%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.5%,
O: 0.0010 to 0.0080%,
Al: 0.001% to 0.025%,
Mg: 0.0020% or less,
Ca: containing 0.0020% or less,
The balance consists of Fe and impurities,
On the surface of the steel sheet, among the oxysulfides having an equivalent circle diameter of 1 μm or more and containing 5% or more of S, the number ratio of (Mn, Cr) S-based oxysulfides is 70% or more, and the number ratio of CaS-based oxysulfides is 70% or more. The number ratio is less than 30%,
A martensitic stainless steel sheet, wherein the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm ² or less on the surface of the steel sheet. in % by mass,
C: 0.10 to 0.60%,
Si: 0.05 to 1.0%,
Mn: [S] × 100% to 1.0% (where [S] is the S content (%)),
P: 0.04% or less,
S: 0.008% or less,
Cr: 11-16%,
Ni: 0.01 to 0.50%,
Cu: 0.01-0.50%,
N: 0.01 to 0.10%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.5%,
O: 0.0010 to 0.0080%,
Al: 0.001% to 0.025%,
Mg: 0.0020% or less,
Ca: 0.0020% or less,
Containing Ta: 0.0005 to 0.01%,
The balance consists of Fe and impurities,
(Mn, Cr) S-based oxysulfides and (Mn, Cr, Ta) S-based oxysulfides among the oxysulfides containing 5% or more of S and having an equivalent circle diameter of 1 μm or more on the surface of the steel sheet The total number ratio is 70% or more, and the CaS-based oxysulfide number ratio is less than 30%,
A martensitic stainless steel sheet, wherein the number density of oxysulfides having an equivalent circle diameter of 5 μm or more and containing 5% or more of S is 0.50 pieces/mm ² or less on the surface of the steel sheet. In place of part of Fe, further, in mass%, Co: 0.05 to 1.00%, Ti: 0.05% or less, Nb: 0.05% or less, B: 0.005% or less, Sn : 0.005 to 0.20% or less, REM: 0.002% or less, containing one or more selected from 0.002% or less. stainless steel plate.

Images